Transplant arteriosclerosis (TA) is a highly-prevalent complication of heart transplantation where diffuse, irreversible stenoses form in graft coronary arteries. Donor specific antibody (DSA) binding MHC on graft endothelial cells (EC) is a strong risk factor for TA, but the mechanism(s) of how DSA causes TA are not well understood. To study this problem, DSA was modeled on EC using high panel reactive antibody (PRA) sera taken from sensitized transplant candidates. PRA sera deposited DSA on EC and activated complement, a system of immune-related proteins that when activated forms pore-like structures called membrane attack complexes (MAC) on cell surfaces. PRA caused MAC assembly on EC which elicited EC activation and enhanced EC immunogenicity of alloimmune CD4+ T cells. These processes had an overall effect of exacerbating TA lesions in human coronary arteries in a humanized mouse model. MAC mediated these effects through a novel effector pathway involving NF-?B-inducing kinase (NIK), a critical mediator of non-canonical NF-?B signaling. This proposal defines mechanisms of how MAC activates NIK in EC, explores how these mechanisms operate in a humanized mouse model for TA, and uses NIK in patient EC as a platform to develop novel diagnostic tests for TA. The Specific Aims proposed herein are unchanged from the original application. Aim 1A and Aim1C, subaims of Specific Aim 1, where endocytosis of MAC was explored as a mechanism for NIK stabilization in vitro and in vivo, were successfully completed during the K99 phase. In the R00 phase, Aim1B of Specific Aim 1, where MAC+Rab5+ subcellular compartments will be isolated to identify how Akt mediates NIK, will be completed. Additionally, in the R00 phase, Aim 2, which explores clinical correlations between NIK and TA in patient biopsy specimens, will be completed. The positive predictive value of NIK expression in two microvessel EC compartments will be assessed in patient samples, and novel markers correlating with TA will be identified using laser capture and RNA profiling of NIK.